Children exposed to radioactive iodine as a consequence of the Chernobyl reactor accident have an increased risk of papillary thyroid carcinomas (PTC). The predominant molecular lesions in these tumors are rearrangements of the RET receptor tyrosine kinase (tk). Here we report on two novel types of RET rearrangement, PTC6 and 7, and describe the fusion products and the ret fused gene (rfg) proteins. Like the other rfg proteins identified so far they are ubiquitously expressed, not membrane-bound and contain coiled coil domains required for constitutive activation of the ret tk domain. In the PTC6 rearrangement the ret tk domain is fused to the aminoterminal part of the human transcription intermediary factor htif 1. In the PTC7 rearrangement the ret tk domain is fused to a novel protein that is strongly related to htif1. Like htif1 it contains a RBCC motif (ring finger, B boxes, coiled coil domain) located in the aminoterminal part and a phd finger and a bromodomain in the carboxyterminal part. Htif1 and related proteins are transcription coactivators for nuclear receptors, thus participating in controlling cellular development, differentiation and homeostasis. This is the first report on their involvement in human thyroid carcinogenesis.